runtime/cgo: declare variable setg_gcc as static

variable setg_gcc in runtime/cgo/*.c should be static, otherwise it
will be mixed with the function of the same name in runtime/asm_*.s or
tls_*.s, which causes an error when building PIE with internal linking
mode.

Fixes #31485

Change-Id: I79b311ffcaf450984328db65397840ae7d85e65d
Reviewed-on: https://go-review.googlesource.com/c/go/+/172498


Reviewed-by: Ian Lance Taylor <iant@golang.org>